1. Field of the Invention
The present invention relates to a mobile communication terminal provided with the dual function of communicating by switching a communication mode between satellite communication and earth-based cellular communication systems.
2. Description of the Prior Art
A mobile communication system (hereafter unified communication system) which comprises a unified plurality of communication modes such as mobile satellite communication (hereafter satellite communication) and earth-based cellular communication (hereafter cellular communication) is comprised by an earth-based service area of small zones in which cellular communication is used and by satellite areas in which satellite communication is used.
In this type of unified communication system, a dual mode terminal (hereafter dual terminal) is used which enables selection of the communication modes of cellular communication or satellite communication. The dual terminal performs cellular communication when positioned in an earth-based service area. Switching of the communication mode is made on the border of an earth-based service area and satellite service area from cellular to satellite communication and thereafter satellite communication is performed in the earth-based service area.
The switching of the communication mode is performed by monitoring the reception levels of signal waves transmitted from base stations in the earth-based service area. FIG. 4 shows a mobile communication device mounted in a communication system as disclosed, for example, in JP-A-10-108262. FIG. 5 shows the relationship between reception level and a communication mode switching standard. In FIG. 4, reference numeral 7 is a radio zone, 8 is a mobile station which displaces in a radio zone, 9 is a base station, 10 is a communication satellite, 11 is a mobile station which displaces outside a radio zone 7. The mobile station 8 which displaces inside the radio zone 7 communicates with the radio base station 9. When the mobile station 8 displaces towards the periphery of the radio zone 7 (the direction of the arrow I), as shown in FIG. 5, the signal level from the base station 9 which is received by the mobile station 8 gradually reduces.
When the mobile station 8 detects a reduction in the signal level of the base station 9, measurement of the signal level of the signal transmitted from the communication satellite 10 is initiated. When the signal level transmitted from the base station 9 falls below a pre-set standard, the transmission quality between mobile station 8 and the base station 9 deteriorates and it becomes difficult to continue communication between the mobile station 8 and the base station 9. In this way, when the signal level from the base station 9 falls below a fixed standard, the mobile station 8 continues communication by switching communications to a communication satellite from the base station 9. Conversely, when a mobile station 11 which is positioned outside the radio zone 7 is displaced towards the radio zone 7 (the direction of the arrow II), the signal level from the base station 9 increases. When the level of the signal exceeds a fixed standard, the communication station switches communication from the communication satellite 10 to the base station 9.
However the threshold value which represents the determination standard for switching communication modes is fixed at a determined value. Thus switching of the communication mode is performed on the basis of the threshold value and the signal level entirely without reference to the requirements of a terminal user. In order to maintain a fixed communication quality, it is desirable that the threshold value is set to a high level and that the communication mode is switched with slight reductions in the received level. On the other hand, when the communication mode is switched frequently, the problem has arisen position registration traffic increases. Furthermore generally satellite communication is relatively more expensive than earth-based systems.
There is strong demand amongst terminal users for the use of cheap cellular communication as opposed to satellite communications. In order to suppress communication costs, the threshold value may be set to a low signal level which enables communication. However low threshold values may result in communication states becoming unstable and communication quality deteriorating. Since the requirements of terminal users with respect to communication costs and communication quality differ depending on respective use environments, a communication terminal device adapted to set a threshold value depending on the use conditions of an individual terminal user would have considerable advantages.
The present invention is proposed to solve the above problems and has a first object of providing a mobile communication terminal device which can set a plurality of communication mode switching threshold values and which can be used to select a desired communication mode switching threshold from a plurality of communication switching threshold values depending on a use condition. The present invention has a second object of providing a mobile communication terminal device which enables the setting of a communication mode switching threshold value in response to the personal requirements of a terminal user.
A mobile communication terminal device according to the present invention comprises a control means which evaluates a communication quality of a received signal by comparing a reception level of a signal transmitted by a satellite and a base station with a fixed threshold value and which switches selectively between a satellite communication mode which performs signal transmission and reception with a satellite and an earth communication mode which performs signal transmission and reception with a base station controlling a cell in a given region, a communication mode input means which displays a menu screen allowing selection of said communication modes by a user, said satellite communication mode and said earth communication mode being prioritized and used out of said communication modes, and said communication mode input means receiving input of a desired communication mode selected by a user, and a threshold value variation means which varies a switching sensitivity of a communication mode with respect to a communication quality of a received signal by varying a threshold value in order to evaluate a communication quality of a received signal in response to a desired communication mode input from said communication mode input means.
A mobile communication terminal device according to the present invention further comprises a threshold value storage means which stores a plurality of threshold values divided into communication modes. A suitable threshold value from among said plurality of threshold values is selected in response to a communication mode input through said communication mode input means and is output to said control means.
A mobile communication terminal device according to the present invention is further characterized in that said communication mode input means displays a communication mode selection screen. The communication selection screen displaying communication modes are selectable by a terminal user and said selectable communication modes are a satellite system priority mode mainly performing communication with a satellite and an earth system communication mode performing communication using a satellite as little as possible. Alternatively said communication mode input means displays a communication mode selection screen displaying a mode prioritizing a low call rate mode or a high quality communication mode prioritizing communication quality.
A mobile communication terminal device according to the present invention is further characterized in that, when a communication mode selected by a terminal user is a mode which prioritizes the use of a low call rate mode, said threshold value variation means determines that an earth system priority mode has been selected and varies a threshold value. When a communication mode is selected which prioritizes a high communication quality, said threshold value variation means determines that a satellite communication mode is selected and varies a threshold value.
A mobile communication terminal device according to the present invention is further characterized in that said control means evaluates a communication quality of a received signal by comparing a fixed threshold value with one of an average value for a received signal value in a unit time, a bit error rate of a received signal or an out-of-synch frequency.